The interfacial reaction of Ta thin films on (100) Si was, investigated by high-resolution transmission electron microscopy. An amorphous layer was observed at the as-deposited Ta/Si interface. A phase of Ta5Si3 was first found to form at the interface between a Ta overlayer and the amorphous layer after annealing at 560 degrees C for 1 h. Annealing at 630 degrees C for 1 h led to the formation of another interlayer due to the outdiffusion of Si between the amorphous layer and Si. The phase in this interlayer transformed from a metastable one to TaSi2 due to annealing at 680 degrees C for 1 h. The first nucleation of Ta5Si3 at the interface between Ta and the amorphous layer implies that the initially formed amorphous layer has a metal-rich composition close to Ta5Si3. The formation of the interlayer between the amorphous layer and Si prior to the nucleation of TaSi2 was considered as a result of a kinetic constraint to favor the nucleation of TaSi2.